The present invention relates generally to the field of variable crystal oscillators and specifically to a variable crystal oscillator suitable for inclusion on an integrated circuit.
Radio frequency mobile communications terminals, such as for example cellular telephones, require an on-board frequency reference for establishing operating channel frequencies. The mobile terminal receives a base station signal and measures the apparent frequency error of the received signal. Since the base stations use highly accurate frequency sources, any error in a received signal is attributed to the mobile terminal""s own frequency reference. Mobile terminals typically utilize crystal oscillator circuits whose frequency is electronically adjustable. Such circuits are known as Voltage Controlled Xtal Oscillators (VCXO). The mobile terminal, upon detecting an apparent frequency error in a received signal, generates a frequency correction signal and adjusts the crystal oscillator to reduce the apparent frequency error. The mobile terminal""s reference oscillator accuracy is thereby adjusted to equal that of the base station by automatic frequency correction (AFC). It is well known in the prior art that the mobile terminal may compute the frequency error digitally, the digital error value then being applied to a digital to analog (D/A) converter to generate a correcting voltage to the VCXO.
Prior art VCXOs are constructed by connecting a quartz crystal resonator in the feedback loop of a sustaining amplifier. A variable-capacitance diode (Varicap or Varactor diode) is associated to the crystal circuit to allow frequency adjustment by applying a voltage to the varactor diode to change its capacitance, and thereby to change the resonant frequency of the circuit formed by the crystal plus the varactor diode.
Great strides have been made in reducing the cost of cellular phones by integrating electrical and logical circuits and functions into silicon or Gallium Arsenide integrated circuits, or chips. However, components such as varactor diodes used in a prior art VCXOs are difficult to integrate, as they require different semiconductor processing steps than the rest of an integrated circuit.
It is known in the art to vary the frequency of a crystal oscillator by varying a phase shift with the feedback loop, rather than using a varactor diode. Furthermore, it is known to produce the varying phase shift by varying the magnitude of a quadrature signal component. However, prior art VCXOs utilizing this approach have various deficiencies. Some do not operate the crystal oscillator in series resonant mode. Others implement control circuits with the undesirable characteristic that the loop gain is dependent on the quadrature signal component that is varied to alter the oscillator""s frequency. This is problematic, particularly when the VCXO must be controlled over a wide frequency range. Furthermore, if the crystal is connected in a relatively high impedance circuit, the Q-factor at series resonance is reduced, thereby degrading the stability and phase noise of the oscillator and rendering it unsuitable for applications in which the crystal frequency must be multiplied up to several GHz, as in modern mobile terminals.
Another known problem with prior art varactor-less VCXOs is that the linearity of the frequency control curve is adversely affected by the crystal""s parasitic shunt capacitance. Some prior art solutions compensate for the crystal parasitic capacitance by employing a variable inductor to linearize the control curve. However, such a component is not suitable for integration.
Still other prior art VCXOs maintain a constant gain under varying quadrature signal components that alter the output frequency, but exhibit a control curve slope that is inversely proportional to the equivalent resistance of the crystal, which is an ill-defined parameter, rendering reliable control difficult.
When integrating the crystal oscillator function with other functions in a few chips to reduce cost, care must be taken to avoid mutual interference between functions, which can occur as signals go in and out of pins on the chip, relative to a common ground. Therefore balanced, or differential, circuits are preferred for reducing unwanted coupling, as is known in the art. Moreover, the crystal in a mobile terminal must be operated at a high Q-factor to obtain low phase noise after multiplying the frequency to the 2 GHz range.
A quartz crystal oscillator comprises a balanced circuit with a quartz crystal resonator device connected in series resonance across a balanced, low-impedance node within a sustaining amplifier. A phase modulator such as a quadrature modulator is included in the feedback loop to allow programming of the loop phase shift thereby to alter the frequency point on the crystal resonance curve at which the circuit oscillates. The in-phase loop signal is hardlimited while the quadrature loop signal component is not hardlimited with the effect that the frequency control curve slope is more accurately defined. An active neutralization of the crystal""s parasitic shunt capacitance is disclosed for obtaining a linear frequency control curve.